Normally written in response to a real life event or online post. Used to express a sense of powerful speechlessness where words will not do the subject justice. It can be used to communicate joy or anger or another extreme emotion.

A random configuration of letters and or numbers that are composed due to a spazzfit.

With fangirls this is generally caused by seeing something so sexy they have no way to process a proper thought and instead seizure on top of their keyboards. Seeing something that reminds them (or proves the existence) of their OTP is will also induce a keyboard smash.

Alternatively the keyboard smash can be induced if something is funny, cracktastic or all of the above all at once.

Moderate fan: Hey, check out this picture of Tom Hiddleston on set! Lol, what a goofball.
Fangirl: ALJFSUGKU:AGSFULUASFGHLUIAAHFDUHIUSHF
Moderate fan: ...Are you okay?
Fangirl: ASDFGHJKL NOOOOOOOOOOOO ASHLUASFUIAFAOFHOUEGUAIEBIBIFHOWAH!!!
Moderate fan: What does that even mean?
Fangirl: It's a keyboard smash, it means my feels were overloaded.